The countries of North America currently import significant portions of their needed liquid hydrocarbons from Asia and Africa. Natural gas is abundant on the North American continent but is often present in remote locations. Although natural gas may be liquified and transported for subsequent use, appropriate compression equipment and transportation are quite expensive. Additionally, there are few economically viable technologies available for converting gaseous hydrocarbons to higher molecular weight liquid form materials. This invention includes a highly effective catalytic step useful in converting methane and other lower alkanes to another, more reactive form which may then be converted to normally liquid hydrocarbons.
It is generally accepted that conversion of methane into a reactive intermediate is the most difficult step in the overall conversion of methane into higher molecular weight hydrocarbons (see, for instance, A. E. Shilov and A. A. Shteinman, "Activation of Saturated Hydrocarbons by Metal Complexes in Solutions", Kinetika i Kataliz, Vol. 18, No. 5, pp. 1129-1145, 1977).
Several documents disclose a variety of methods for activating methane to produce other higher molecular weight materials.
Mobil Oil Corporation is assignee in several U.S. patents using sulfur or certain sulfur-containing compounds as the reactants in non-catalytic reactions with methane to produce methyl intermediates which can then be converted to higher molecular weight hydrocarbons.
In U.S. Pat. No. 4,543,434 Chang teaches a process using the following steps: ##STR1## where "[CH.sub.2 ]" is a hydrocarbon having at least two carbon atoms.
Another Mobil disclosure (U.S. Pat. No. 4,864,073 to Han et al.) suggests a carbonyl sulfide-based process in which methane and carbonyl sulfide are contacted in the presence of ultraviolet light under conditions sufficient to produce CH.sub.3 SH. No other reaction initiators are said to be present. The reaction scheme is shown to be: ##STR2## The selectivity of the first reaction is said to be high, i.e., around 81%; however, the conversion appears to be quite low.
A disclosure similar to that in Chang is found in Mobil's U.S. Pat. No. 4,864,074 to Han et al. As in Chang, the methane is contacted with sulfur. The process conditions are changed, however, so that either CS.sub.2 or CH.sub.3 SH is formed. These sulfur compounds may then be converted in the presence of the preferred HZSM-5 zeolite catalyst to produce hydrocarbons having two or more carbon atoms. Also, as was the case with Chang, the step of contacting the methane to produce a methyl-sulfur compound is performed in the absence of a catalyst.
Other methods are known for producing substituted methanes which are suitable for further reaction to heavier hydrocarbons. A thermal methane chlorination process is shown in U.S. Pat. No. 4,804,797 to Minet et al. A similar process is disclosed in U.S. Pat. No. 3,979,470 to Firnhaber et al. although a preference for C.sub.3 hydrocarbon feeds is expressed.
One method shown in U.S. Pat. No. 4,523,040 to Olah utilizes either a solid strongly acidic catalyst or a supported Group VIII metal (particularly platinum and palladium) in the gas phase halogenation of methane to produce methyl halides. The patent indicates that monohalides are produced in 85% to 99% selectivity. Olah suggests that subsequent or concurrent catalytic hydrolysis produces methyl alcohol and/or dimethyl ether. Production of methyl oxy-esters is not shown.
The reaction of methane with palladium (II) acetate in trifluoroacetic acid to effect the trifluoroacetoxylation of methane is shown in Sen et al., "Palladium (II) Mediated Oxidative Functionalization of Alkanes and Arenes", New Journal of Chemistry (1989), Vol. 13, No. 10-11, pp. 756-760. A yield of 60% based on palladium was reported when the process was practiced using methane as the reactant. Consequently, the reaction utilized palladium as a reactant and not as a catalyst. Palladium was shown in the article to be catalytic in the trifluoroacetoxylation of an arene.
The Sen et al. article has been criticized in Vargaftik et al., "Highly Selective Partial Oxidation of Methane to Methyl Trifluoroacetate", Journal of the Chemical Society, Chemical Communications (1990), pp. 1049-1050, to the extent that the results were not found to be reproducible. Vargaftik et al. discloses the catalytic oxy-esterification of methane with cobalt but shows that palladium is not even suitable for stoichiometric methane oxidation in that process. Less than 0.1% yield of methyl trifluoroacetate based on palladium (II) trifluoroacetate was obtained.
Our inventive process is the first of which we are aware in which a lower alkane is oxidized to a an oxy-ester intermediate using a Group VIII noble metal catalyst, which intermediate is suitable for further conversion into heavier liquid hydrocarbons.